Currently, strong interest is directed to a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern features. A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
Patent Document 1 discloses a process involving forming a first pattern of a positive resist material through exposure and alkaline development, rendering the first pattern insoluble in organic solvent and alkaline developer with the aid of acid and heat, coating another positive resist material thereon, and forming a second pattern through exposure and alkaline development. Patent Document 2 discloses a process involving forming a first pattern of a positive resist material through exposure and alkaline development, rendering the first pattern insoluble in organic solvent and alkaline developer with the aid of light irradiation and heat, coating another positive resist material thereon, and forming a second pattern through exposure and alkaline development. These processes belong to a double patterning process involving the steps of insolubilizing a first positive resist pattern and combining it with a second positive pattern.
As compared with the line pattern, the trench or hole pattern is difficult to reduce its size. If an attempt is made to form fine holes according to the prior art method by combining a positive resist film with a hole pattern mask and effecting under-exposure, the exposure margin is extremely narrowed. It is then proposed to form holes of larger size and shrink the developed holes by thermal flow, RELACS® or other techniques. However, there is a substantial difference between the pattern size as developed and the pattern size as shrunk, giving rise to the problem that a greater shrinkage leads to a lower control accuracy. With the hole shrinking method, the hole size can be shrunk, but the pitch cannot be narrowed.
Holes can be shrunk using the direct self assembly (DSA) technology. A typical DSA material is a styrene-methacrylate block copolymer. The DSA technology can shrink a hole pattern if holes are of true circle. When the DSA technology is applied to elongated holes or trench patterns, juxtaposed holes are formed. It is a drawback that shrinkage occurs while the original shape is deformed.
Recently a highlight is put on the organic solvent development again. It would be desirable if a very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone exposure. To this end, a positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. An attempt to double a resolution by combining two developments, alkali development and organic solvent development is under study.
As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such pattern forming process is described in Patent Document 3.
An attempt to form elongated holes via negative development results in elliptic shape. This is due to the influence of optical interference. The elliptic hole pattern is undesired when it is intended to form a rectangular hole pattern.
Non-Patent Document 1 proposes a process for reducing the pitch to one half by heating a first positive resist pattern for insolubilizing it in organic solvent and alkaline developer, coating a second resist material, and forming a second positive pattern between the first positive pattern features. If the second pattern is orthogonal to the first pattern, a hole pattern can be formed. If the pitch is changed between the first and second lines, rectangular holes can be formed. However, since both the first and second steps use positive resist materials, formation of a narrow trench pattern is disadvantageous from the standpoint of optical contrast. Additionally, the corner between orthogonally intersecting sides is rounded.
Patent Document 4 proposes an image reversal technology involving forming a positive resist pattern of positive resist material via alkaline development, heating the positive resist pattern for insolubilizing it in organic solvent while maintaining alkaline development capability, coating a film of low alkaline solubility, and effecting development to dissolve only the surface layer of the low alkaline solubility film while leaving the majority of film, and to dissolve the positive resist pattern in alkaline developer. The means of insolubilizing the film in organic solvent relies on a base polymer derived from a methacrylate monomer having 7-oxanorbornane.
When the super-resolution technology is applied to repeating dense patterns, the pattern density bias between dense and isolated patterns, known as proximity bias, becomes a problem. As the super-resolution technology used becomes stronger, the resolution of a dense pattern is more improved, but the proximity bias is exaggerated. In particular, an increase of proximity bias in a hole pattern poses a serious problem. One common approach taken to suppress the proximity bias is by biasing the size of a mask pattern. Since the proximity bias varies with properties of a photoresist composition, specifically dissolution contrast and acid diffusion, the proximity bias of a mask varies with the type of photoresist composition. For a particular type of photoresist composition, a mask having a different proximity bias must be used. This adds to the burden of mask manufacturing. Then the pack and unpack (PAU) method is proposed in Non-Patent Document 2: Proc. SPIE Vol. 5753, p 171 (2005), which involves strong super-resolution illumination of a first positive resist to resolve only a dense hole pattern, coating the first positive resist pattern with a negative resist film material in alcohol solvent which does not dissolve the first positive resist pattern, exposure and development of an unnecessary hole portion to close the corresponding holes, thereby forming both a dense pattern and an isolated pattern. One problem of the PAU method is misalignment between first and second exposures, as the authors point out in the report. The hole pattern which is not closed by the second development experiences two developments and thus undergoes a size change, which is another problem. While this method uses a positive resist material in the first exposure step, the use of a positive resist material to form hole patterns suffers from low resolution and poor dimensional control as discussed above.